[EPIC] Define and implement CLI update, auto‑update, and channel management for azd

[kristenwomack]

Overview

Today when typing an azd command and there is a new version available, the customer gets a message to copy/paste and run the script to install the lastest azd version. Often customer try azd upgrade to get the latest version, but we don't have that feature (yet). The terminal output with instructions is helpful but could be easier and smoother.

This Epic focuses on delivering a clearer and more intuitive update experience for azd—starting simple and iterating through a crawl / walk / run approach getting to auto updates (that don't break pipeline automation).

The goal is to:

• Make it easy for users to update azd intentionally (azd update)
• Experiment with opt‑in auto‑update safely (starting with Insiders)
• Preserve user control (opt‑out, pin versions, stable vs Insiders)
• Avoid noisy or disruptive behavior
• Respect platform realities (Windows MSI, macOS Homebrew, scripts)

We will start with Insiders builds to test edge cases, learn from real usage, and incrementally evolve toward a default‑safe, low‑friction update model.